This invention relates to downhole resistivity logging methods and apparatus and, more particularly, to downhole resistivity logging apparatus for remote sensing of resistive or conductive anomalies at a distance from the borehole.
Electrical logging of boreholes is well known and has become a standard practice. The measurement of electrical resistivity in a borehole has been used to determine formation characteristics, both near and far from the borehole. As the resistivity measuring tools and apparatus have been improved, the depth of measurement into the formation has been increased in an effort to locate possible petroleum-containing formations which have been missed by the wellbore itself.
One example of an electrical logging device intended primarily for the location of a salt dome missed by the borehole is found in U.S. Pat. No. 3,256,480 issued to Runge et al. on June 14, 1966. The logging device described in this patent is known as a normal electrode configuration having a current input electrode at the lowest point of the device and a series of sensing electrodes spaced at approximately exponentially increasing spacings above the current electrode and over a distance of from 200 to 1,000 feet. The return current electrode in this normal electrode configuration is an earth contact at the surface of the earth. The disclosed device typically has six sensing electrodes and is used to detect a salt dome by comparing the apparent resistivity measured at each of the electrodes to an averaged or composite resistivity generated from short-spaced electrode measurements or some other short-range logging device. A difference in the apparent resistivity measured by a particular electrode and the average resistivity is an indication of a salt dome at a distance corresponding to the particular electrode spacing.
Another resistivity logging device also intended for locating a salt dome is disclosed in U.S. Pat. No. 3,697,864 issued to Runge on Oct. 10, 1972. The logging cable disclosed in this patent is essentially a very long version of a common four-point resistivity measuring device. It has two current electrodes spaced apart on the order of 1,000 feet and two voltage sensing electrodes spaced symmetrically between the two current electrodes. Voltage measurements taken by the two sensing electrodes in this long four-point array are used with a well-known mathematical formula to generate an apparent resistivity measurement which is again compared to a composite resistivity measurement derived from short-spaced electrode measurements or other short-range electrical resistivity measurements. If the two measurements differ significantly, a large resistivity anomaly such as a salt dome is indicated at a distance from the wellbore on the order of the electrode spacings.
Yet another resistivity measurement device used primarily to detect horizontal anomalies such as pools of oil near vertical anomalies such as salt domes is disclosed in U.S. Pat. No. 3,778,701 issued to Runge on Dec. 11, 1973. This apparatus has a single downhole current electrode and a second current electrode contacting the earth's surface. It also has two sensing electrodes with one spaced above the downhole current electrode and the other below it by the same distance. With this device, the voltage between the two sensing electrodes is essentially zero, except when the device is near a horizontal anomaly such as a pool of oil.
A mathematical description of multiple-point electrode logging devices is provided by the reference: "A Systematic Theoretical Description of Multiple-Point Electrode Logging Devices. The Zoom-Log," by J. M. Lepa, W. F. Kozik, and S. Plewa; Acta Geophys. Pol., Vol. 19, No. 2, pp. 149-165, 1971. This reference provides mathematical descriptions of logging devices having from one to five current electrodes. It additionally illustrates the use of superposition of potential fields to analyze these various logging devices. As proposed by this reference, it is possible to generate the resistivity measurements of various complex logging devices by appropriately combining readings taken with a number of simpler logging devices. The "zoom log" proposed by this reference is a complicated five current electrode device which can provide a depth of investigation which is variable by means of current ratio adjustment. It is suggested that a more practical means of achieving the desired results is to provide six measurements taken by simpler devices and to mathematically combine these six measurements in a computer program to obtain the "zoom log" measurements.
In those electrical logging devices which have only one downhole current electrode and a return electrode at the earth's surface, large volumes of the earth are energized. Consequently, voltages measured at sensing electrodes reflect not only the influence of anomalous structures of interest but also of subsurface materials of no economic significance. This amounts to a lack of resolution in such devices.
In those electrical logging devices which have both current electrodes downhole and relatively close together, the volume of the earth affected by the energizing current is reduced. This arrangement causes another problem in that there must generally be a fixed spacing between the current electrodes and the data obtained, therefore, reflects variations in resistivity corresponding to a fixed depth of penetration as determined by the current electrode spacing. It is possible to have more than one pair of downhole electrodes, as is done in the above references U.S. Pat. No. 3,697,864, but this requires the addition of electrodes and conductors capable of carrying the survey current so that it is not practical to expand the range of investigation to any great extent in this manner.
The "zoom log" disclosed in the above referenced technical article can provide a continuous curve of resistivity as a function of depth of investigation, but requires a complex logging tool in terms of number of current electrodes and regulated variable current sources. The alternate "zoom log" arrangement is apparently either a logging tool which can simultaneously provide six simpler logging measurements using a plurality of current inputs or six separate logging devices which must be run in the borehole on separate logging runs.
Accordingly, an object of the present invention is to provide an improved resistivity logging device and method for geoelectric remote sensing.
Another object of the present invention is to provide a simple resistivity logging device having a single downhole current electrode.
Another object of the present invention is to provide a resistivity logging device and method for measuring formation resistivity over a broad and virtually continuous range of depths of investigation.
Another object of the present invention is to provide a downhole resistivity logging device and method which can provide formation resistivity measurements over an essentially continuous range of borehole locations.
Yet another object of the present invention is to provide a downhole resistivity logging device and method for making high resolution measurements of the distance to and shape and resistivity of geologic structures either penetrated or missed by the borehole.
These and other objects are achieved by providing a resistivity logging cable comprising a single current electrode at the center of the logging device and two arrays of voltage sensing electrodes spaced symmetrically above and below the current electrode. A return path for the current electrode is provided by a surface earth contact. The voltages between adjacent pairs of sensing electrodes are recorded while a very low frequency current is supplied to the formation from the current electrode. Apparent resistivity is then determined using the four-point resistivity equation by algebraically adding the voltage difference between a pair of sensing electrodes above the current electrode with the voltage difference between a pair of electrodes the same distance below the current electrode, but measured at a later time when the lower electrode pair is moved to the same position as the upper electrode pair had occupied. By thus composing the recorded measurements, the advantages of having two downhole electrodes are achieved in a device having only one downhole electrode.